1. Field of the Invention
The present invention relates to a catalyst for purifying exhaust gases which are discharged from internal-combustion engines, boilers, gas turbines or the like and contain nitrogen oxides together with excess oxygen, and to a method for purifying the exhaust gases.
2. Description of the Prior Art
Nitrogen oxides (NOx) emitted from internal-combustion engines and the like may cause photochemical smog and acid rain, and elimination of NOx from their sources is urgently needed.
For elimination of NOx from exhaust gases from large-scale fixed sources such as electric power plants, heretofore, a selective catalytic reduction method has been used in which ammonia is added to the exhaust gas and a TiO.sub.2 --V.sub.2 O.sub.5 catalyst is used. For eliminating NOx from exhaust gases from gasoline engines of automobiles and the like, on the other hand, a three-way catalyst (TWC) method has been used in which air-fuel ratio is controlled into the vicinity of the stoichiometric value (A/F=14.6) and a Pt--Rh/Al.sub.2 O.sub.3 catalyst is used to thereby eliminate NOx simultaneously with carbon monoxide (CO) and hydrocarbons (HC).
Meanwhile, in order to prevent the earth's surface temperature from rising due to air pollution, it has become required in recent years to control emission of carbon dioxide (NOx) and to put into practical use a lean-burn gasoline engine in which gasoline is burned at a air-fuel ratio on the "leaner" side of the stoichiometric value. Exhaust gases from such lean-burn gasoline engines cannot be treated effectively with the three-way catalyst method.
In addition, diesel engines inherently are lean-burn engines, and elimination of NOx from their exhaust gases is also a problem of great urgency.
These lean-burn gasoline engines, diesel engines and other engines of the lean burn type are generically called lean-burn engines.
Exhaust gas from a lean-burn engine contains oxidants, oxygen (O.sub.2) and NOx in excess of the stoichiometrically necessary quantity for complete oxidation of hydrocarbons (HC) (the term "hydrocarbons" herein refers to not only hydrocarbons in a narrow sense but also partially oxidized products thereof such as oxygenated hydrocarbons, for example, alcohols and ketones) which are products of incomplete oxidation of the fuel, CO and reducing components such as hydrogen (H.sub.2), etc. Specifically, the exhaust gas may contain oxygen in such a high concentration as to give an air-fuel ratio A/F Of 17 or more. It has long been an unsolved problem to achieve selective elimination of NOx from exhaust gas in the presence of the excess oxygen without adding any special reducing agents such as ammonia.
For elimination of NOx from an exhaust gas which also contains excess oxygen, methods have been proposed in recent years in which NOx is selectively reduced in the presence of hydrocarbons by using a transition metal such as copper, cobalt, iron or the like supported through ion-exchange on an aluminosilicate [U.S. Pat. No. 4,297,328 and Japanese Pre-examination Patent Publication (KOKAI) No. 63-100919], a metalloaluminosilicate [Japanese Pre-examination Patent Publication (KOKAI) Nos. 3-127628 and 3-229620], a silicoaluminophosphate [Japanese Pre-examination Patent Publication (KOKAI) No. 1-112488] or the like as a catalyst. However, the transition-metal ion-exchange zeolite catalyst and the like just mentioned have a fatal drawback that they undergo irreversible deactivation in a short time when exposed to a water vapor-containing exhaust gas at a temperature of 650.degree. to 700.degree. C.; therefore, none of these catalysts have been successfully put to practical use.
On the other hand, metallosilicate catalysts in which a catalytically active component is not placed in ion-exchange sites but introduced into the skeleton of a crystalline silicate by isomorphous replacement have been proposed, for example, copper silicate catalysts [Japanese Pre-examination Patent Publication (KOKAI) No. 2-265649]; iron silicate, cobalt silicate or nickel silicate catalysts (A collection of papers for lectures at the 65th annual spring meeting of the Chemical Society of Japan, I, lecture No. 2F138), aluminorhodium silicate catalysts [Japanese Pre-examination Patent Publication (KOKAI) No. 5-76770] and the like. However, these catalysts are much lower than ion-exchange type catalysts in activity for selective reduction of nitrogen oxides in the practical space velocity range (GHSV.gtoreq.10,000/hr).
Further methods proposed for eliminating NOx from an exhaust gas in the presence of excess oxygen employ a supported noble metal catalyst which is expected to exhibit high heat resistance. For example, a method of eliminating NOx from an exhaust gas containing excess oxygen by use of a catalyst comprised of iridium (Ir) supported on a porous inorganic oxide such as alumina (Al.sub.2 O.sub.3) has been disclosed [Japanese Patent Publication (KOKOKU) Nos. 56-54173 and 57-13328 and U.S. Pat. No. 4,039,622 (1977)]. In the above literature, however, there is exemplified this type of catalyst showing catalytic performance only where oxygen concentration of exhaust gas is not more than 3%, or A/F is less than 17, and its NOx elimination performance is insufficient for the lean-burn engine exhaust gases of current concern which contain oxygen in such excess amounts that A/F is not less than 17. Thus, the greatest problem in the method of purifying exhaust gas by using a supported noble metal catalyst in an atmosphere containing excess oxygen is that due to the high oxidizing activity inherent to noble metal, hydrocarbons are preferentially oxidized by oxygen in the catalyst's actual service temperature range of 350.degree. to 500.degree. C., particularly in a high oxygen concentration region, so that selectivity in reduction of NOx is unsatisfactory.